The project proposes the study of perceptual orientation by means of four approaches: 1. Algorithm processing. A processing model is proposed that serves to account for a variety of constancies of object orientation. Two sample algorithms are derived relating body and object orientation for median and lateral plane tilts analogous to the size-distance invariance hypothesis. Several tests of the algorithm processing approach are described. 2. Relational determinants. These refer to the main lines of the visual field which may serve as a frame of reference for the orientation of objects as well as of the self. Proposed studies include the effects of frame tilt, frame size, and retinal locus on the rod and frame effect (RFE) and on induced body tilt (IBT). 3. Egocentric orientation. Reference is made to the perceived orientation of objects with respect to the self in contrast with orientation perceived as relative to the direction of gravity or with respect to other objects. 4. Adaptation to optically rotated visual fields. Studies are proposed on the locus of the adaptation aftereffect, on the relation between the vestibular ocular reflex (VOR) and adaptation to optical tilt, and on tilt adaptation as a feedback-controlled process.